1. Technical Field
The present invention relates to a pellicle for lithography for use as a dustproof cover for a photomask, a reticle or the like in the manufacture of a semiconductor device such as a large-scale integrated circuit, a very-large-scale integrated circuit (LSI) or the like or a liquid crystal display panel.
2. Description of the Related Art
In the case of manufacturing a semiconductor device such as a large-scale integrated circuit (LSI), a very-large-scale integrated circuit or the like, or a liquid crystal display panel, a semiconductor wafer or a mother substrate for a liquid crystal display panel is irradiated with exposure light via an exposure stencil such as a photomask or a reticle (hereinafter collectively referred to as “photomask” in this specification), whereby a pattern of the photomask is transferred onto the semiconductor wafer or the mother substrate for a liquid crystal display panel so that a pattern of the semiconductor device or the liquid crystal display panel is formed.
Therefore, when the semiconductor wafer or the liquid crystal display panel mother substrate is irradiated with exposure light via the photomask, any foreign materials such as dust particles stuck to the photomask, reflect or absorb the exposure light, so that not only does the pattern transferred onto the semiconductor wafer or the liquid crystal display panel mother substrate become deformed and the edge portion of the pattern become blurred but underlying surface is also blackened by soiling, whereby the size, quality, appearance and the like of the semiconductor wafer or the liquid crystal display panel mother substrate are degraded. As a result, the pattern of the photomask cannot be transferred onto the semiconductor wafer or the liquid crystal display panel mother substrate in the desired manner and the performance of the semiconductor wafer or the liquid crystal display panel mother substrate is lowered, whereby the yield ratio of the semiconductor wafer or the liquid crystal display panel mother substrate is inevitably reduced.
In order to solve these problems, the operation of exposing the semiconductor wafer or the liquid crystal display panel mother substrate is generally conducted in a clean room. However, even in such a case, since it is extremely difficult to completely prevent foreign materials from attaching to the surface of the photomask, the operation of exposing the semiconductor wafer or the liquid crystal display panel mother substrate is usually conducted with a dust proof cover, called a pellicle, and having a high transmittance with respect to the exposure light, mounted on the surface of the photomask.
Generally, a pellicle is manufactured by adhering a pellicle membrane made of a material having a high transmittance with respect to exposure light, such as a cellulose based resin like nitrocellulose and cellulose acetate, fluoride system resin or the like, to one surface of a pellicle frame made of aluminum, stainless steel, polyethylene or the like by applying a good solvent for the material of the pellicle membrane onto the one surface of the pellicle frame and adhering the air-dried pellicle membrane onto the one surface of the pellicle frame, or adhering the pellicle membrane onto the one surface of the pellicle frame using an adhesive agent such as acrylic resin, epoxy resin, fluorine resin or the like, forming an agglutinant layer composed of polybutene resin, polyvinyl acetate resin, acrylic resin, silicone resin or the like and adapted for adhering the photomask to the pellicle frame on the other surface of the pellicle frame, and providing a liner for protecting the agglutinant layer on the agglutinant layer (See Japanese Patent Application Laid Open No. 58-219023, U.S. Pat. No. 4,861,402, Japanese Patent Publication No. 63-27707 and Japanese Patent Application Laid Open No. 7-168345, for example).
The pellicle is mounted on the photomask so as to surround a pattern region formed on the surface of the photomask, whereby the pattern region formed on the surface of the photomask and the region outside of the pellicle are isolated from each other by the pellicle so that dust particles present outside of the pellicle are prevented from adhering to the pattern region formed on the surface of the photomask.
In the case of mounting the thus constituted pellicle on the surface of the photomask and exposing the semiconductor wafer or the liquid crystal display panel mother substrate via the photomask, since foreign materials such as dust particles attach to the surface of the pellicle and do not directly attach to the surface of the photomask, it is possible to avoid the effect of foreign materials such as dust particles by projecting exposure light in such a manner that the light is focused on the pattern formed on the photomask.
Conventionally, an ultraviolet ray such as a G-line UV (436 nm wavelength) and an I-line UV (365 nm wavelength) has been used as the light for exposing an LSI. However, it has recently become necessary to use light having a shorter wavelength as the exposure light because LSI integration density has increased and circuit geometry become finer. Thus, a krypton fluoride (KrF) excimer laser beam (248 nm wavelength) is first used and an argon fluoride (ArF) excimer laser beam (193 nm wavelength) is then used.
However, in the case where the photomask on which a pellicle is mounted is exposed to an argon fluoride (ArF) excimer laser beam (193 nm wavelength) for a long time, solid-like foreign materials gradually separate out on the pattern region of the photomask surrounded by the pellicle.
It is considered that this problem of solid-like foreign materials gradually separating out on the pattern region of the photomask surrounded by the pellicle in the case where the photomask on which the pellicle is mounted is exposed for a long time is caused by the presence of ionic residue on the photomask, an ionic gas or an organic gas contained in the environment where the photomask is used, or reaction of an ionic gas or an organic gas emitted from the pellicle with an argon fluoride (ArF) excimer laser beam.
Among these problems, most of the organic gas emitted from the pellicle is emitted from an agglutinant agent contained in the agglutinant layer for fixing the pellicle to the photomask, so the agglutinant agent is often degassed by heating or keeping it under reduced pressure in advance.
The organic gas emitted from the agglutinant agent contained in the agglutinant layer depends on the kind of agglutinant agent and the higher the absorbing capacity of the agglutinant agent with respect to an argon fluoride (ArF) excimer laser beam (193 nm wavelength) is, the higher the risk of solid-like foreign materials separating out on the pattern region of the photomask is. Examples of an organic gas having a high absorbing capacity with respect to the argon fluoride (ArF) excimer laser beam (193 nm wavelength) include a hydrocarbon based gas such as an aromatic gas having a pi (π) electron. It is further known that an amine system gas, an alcohol system gas or an aldehyde system gas also has a high absorbing property with respect to the argon fluoride (ArF) excimer laser beam (193 nm wavelength).
On the other hand, it has been found that even if the emission of an organic gas from the agglutinant agent contained in the agglutinant layer is suppressed, in the case where the pellicle is used for a long time and exposed to the argon fluoride (ArF) excimer laser beam, the argon fluoride (ArF) excimer laser beam scattered by the pellicle is projected onto the agglutinant layer, whereby the agglutinant agent contained in the agglutinant layer is decomposed to emit an organic gas and the organic gas reacts with the argon fluoride (ArF) excimer laser beam in a closed space defined by the pellicle and the photomask so that solid-like foreign materials separate out on the pattern region of the photomask surrounded by the pellicle.
It has further been observed that when the argon fluoride (ArF) excimer laser beam passes through the pellicle and the photomask, an ozone gas is generated in the closed space defined by the pellicle and the photomask and the thus generated ozone gas promotes the degradation of the agglutinant agent contained in the agglutinant layer, whereby a decomposition gas is emitted.
These problems arise not only in the case where an argon fluoride (ArF) excimer laser beam (193 nm wavelength) is used as exposure light but also in the case where a krypton fluoride (KrF) excimer laser beam (248 nm wavelength) is used as exposure light.
In addition, as explained above, in the case where the pellicle is used for a long time, since failure occurs owing to, for example, separation of solid-like foreign materials onto the pattern region of the photomask, it is necessary after passage of a predetermined time period to remove the solid-like foreign materials separated out on the photomask by cleaning the photomask and replacing the pellicle with a new one, and in the case where the argon fluoride (ArF) excimer laser beam (193 nm wavelength) is used as the exposure light, it is necessary to replace the pellicle with a new one after a shorter time than in the case where the krypton fluoride (KrF) excimer laser beam (248 nm wavelength) is used as the exposure light. However, when the argon fluoride (ArF) excimer laser beam (193 nm wavelength) is projected onto the agglutinant agent contained in the agglutinant layer or the agglutinant agent contained in the agglutinant layer is exposed to an ozone gas, the degradation of the agglutinant agent is promoted and it becomes difficult to peel the photomask off the pellicle frame.